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Abstract:

A medium voltage adjustable frequency drive includes an input isolation
transformer having a three-phase input and a three-phase output, a
converter having a three-phase input electrically connected to the
three-phase output of the input isolation transformer and an output
providing a direct current bus, an inverter having an input electrically
connected to the output of the converter and a three-phase output, and a
pre-charge circuit. The pre-charge circuit includes a ferro-resonant
transformer circuit having a primary winding structured to input a low
voltage and a secondary winding structured to output a medium voltage and
provide a constant current source. The pre-charge circuit also includes a
medium voltage diode bridge having an input receiving the medium voltage
from the secondary winding of the ferro-resonant transformer circuit and
an output structured to provide the constant current source to the direct
current bus.

Claims:

1. A pre-charge circuit for a direct current bus of a voltage source
inverter, said pre-charge circuit comprising:a ferro-resonant transformer
circuit comprising a primary winding structured to input a low voltage
and a secondary winding structured to output a medium voltage and provide
a constant current source; anda medium voltage diode bridge circuit
comprising an input receiving the medium voltage from the secondary
winding of said ferro-resonant transformer circuit and an output
structured to provide said constant current source to said direct current
bus.

2. The pre-charge circuit of claim 1 wherein said ferro-resonant
transformer circuit further comprises a transformer including the primary
winding and the secondary winding, and a capacitor tuning said
transformer.

3. The pre-charge circuit of claim 2 wherein the secondary winding of said
transformer includes a plurality of taps and a selector electrically
connecting one of said taps to the input of said medium voltage diode
bridge circuit.

4. The pre-charge circuit of claim 3 wherein said selector is a jumper.

5. The pre-charge circuit of claim 2 wherein the primary winding has a
number of first turns; wherein the secondary winding has a plurality of
second turns; and wherein a ratio of said plurality of second turns to
said number of first turns is at least about 2400.

6. The pre-charge circuit of claim 5 wherein said ratio is selected from
the list consisting of about 2400, about 3700 and about 6000.

7. The pre-charge circuit of claim 2 wherein said ferro-resonant
transformer circuit is structured to disable said constant current source
when said transformer saturates.

8. The pre-charge circuit of claim 1 wherein the output of said medium
voltage diode bridge circuit includes a first conductor electrically
connected in series with a first medium voltage fuse and a second
conductor electrically connected in series with a second medium voltage
fuse.

9. The pre-charge circuit of claim 8 wherein said first conductor and said
first medium voltage fuse are structured to charge a first capacitor; and
wherein said second conductor and said second medium voltage fuse are
structured to charge a second capacitor.

10. A voltage source inverter comprising:an input isolation transformer
comprising an input and an output;a converter comprising an input
electrically connected to the output of said input isolation transformer
and an output providing a direct current bus;an inverter comprising an
input electrically connected to the output of said converter and an
output; anda pre-charge circuit comprising:a ferro-resonant transformer
circuit comprising a primary winding structured to input a low voltage
and a secondary winding structured to output a medium voltage and provide
a constant current source, anda medium voltage diode bridge circuit
comprising an input receiving the medium voltage from the secondary
winding of said ferro-resonant transformer circuit and an output
structured to provide said constant current source to said direct current
bus.

11. The voltage source inverter of claim 10 wherein the input of said
input isolation transformer is a three-phase input and the output of said
input isolation transformer is a three-phase output; wherein the input of
said converter is a three-phase input and the output of said converter
comprises a positive output, a first neutral and a negative output;
wherein the input of said inverter comprises a positive input
electrically connected to said positive output, a second neutral
electrically connected to said first neutral, and a negative input
electrically connected to said negative output; wherein the output of
said inverter is a three-phase output structured to power a three-phase
rotating electrical apparatus; and wherein the output of said medium
voltage diode bridge circuit comprises a positive output electrically
connected to said positive input and a negative output electrically
connected to said negative input.

12. The voltage source inverter of claim 11 wherein the input of said
inverter further comprises a first capacitor electrically connected
between said positive input and said second neutral and a second
capacitor electrically connected between said negative input and said
second neutral.

13. The voltage source inverter of claim 10 wherein said ferro-resonant
transformer circuit further comprises a transformer including the primary
winding and the secondary winding, and a capacitor tuning said
transformer.

14. The voltage source inverter of claim 13 wherein the secondary winding
of said transformer includes a plurality of taps and a selector
electrically connecting one of said taps to the input of said medium
voltage diode bridge circuit.

15. The voltage source inverter of claim 14 wherein said selector is a
jumper.

16. The voltage source inverter of claim 13 wherein the primary winding
has a number of first turns; wherein the secondary winding has a
plurality of second turns; and wherein a ratio of said plurality of
second turns to said number of first turns is at least about 2400.

17. The voltage source inverter of claim 16 wherein said ratio is selected
from the list consisting of about 2400, about 3700 and about 6000.

18. The voltage source inverter of claim 13 wherein said ferro-resonant
transformer circuit is structured to disable said constant current source
when said transformer saturates.

19. The voltage source inverter of claim 10 wherein the output of said
medium voltage diode bridge circuit includes a first conductor
electrically connected in series with a first medium voltage fuse and a
second conductor electrically connected in series with a second medium
voltage fuse.

20. The voltage source inverter of claim 10 wherein said ferro-resonant
transformer circuit further comprises a pair of contactor contacts
electrically connected in series with said primary winding.

21. The voltage source inverter of claim 10 wherein said secondary winding
is a first secondary winding; and wherein said ferro-resonant transformer
circuit further comprises:a transformer including the primary winding,
the first secondary winding and a second secondary winding, anda
capacitor electrically connected across the second secondary winding and
being structured to tune said transformer.

[0004]A voltage source inverter, such as a medium voltage adjustable
frequency drive, powers a motor, such as an induction or synchronous
motor, or a generator, with a suitable medium voltage.

[0005]Another example of a voltage source inverter is a variable frequency
drive (VFD), which controls the rotational speed of an alternating
current (AC) electric motor by controlling the frequency of the
electrical power supplied to the motor. A VFD is a specific type of
adjustable speed drive. VFDs are also known as adjustable frequency
drives (AFDs), variable speed drives (VSDs), AC drives, microdrives or
inverter drives. Since the voltage is varied along with the frequency,
these are sometimes also called VVVF (variable voltage variable
frequency) drives.

[0006]As shown in FIG. 1, the main components of a voltage source
inverter, such as the example medium voltage adjustable frequency drive
2, include a number of input isolation transformers 4 (two example
transformers 4 are shown), a converter, such as a number of rectifier
bridge assemblies 6 (two example bridge assemblies 6 are shown), a direct
current (DC) bus 8, associated DC bus capacitors 10,12, and an inverter
14. The DC bus capacitors 10,12 store energy and are the voltage source
for the inverter 14. As is well known, an inverter, such as 14, is an
electronic circuit that converts DC to AC. An inverter performs the
opposite function of a rectifier and converts a DC voltage into a
variable voltage, variable frequency AC voltage.

[0007]In order to turn on the example medium voltage adjustable frequency
drive 2, the DC bus capacitors 10,12 must first be charged. This process
is called "pre-charge". Without pre-charge, the inrush current to the
medium voltage adjustable frequency drive 2 is relatively very large, may
damage the number of rectifier bridge assemblies 6, and also may cause
upstream protective relays (not shown) to operate and trip main circuit
breakers (not shown).

[0008]In order to reduce the inrush current when energizing the number of
input isolation transformers 4 and the number of rectifier bridge
assemblies 6 that power the medium voltage adjustable frequency drive 2,
the DC bus capacitors 10,12 are pre-charged. As shown in FIG. 1, a known
proposal for pre-charging uses reactors (or resistors) 16 in series with
the input isolation transformers 4 to limit the inrush current.
Alternatively, resistor(s) can be placed in series with the output of the
transformers 4, or inductor(s) can be placed in series with the
transformers 4 to limit inrush. When the DC bus capacitors 10,12 are
sufficiently charged, the reactors (or resistors) 16 are removed from the
circuit by a medium voltage rated contactor 18.

[0009]A disadvantage of such known methods of reducing inrush current is
the relatively large size and cost of the reactors (or resistors) 16 and
the medium voltage rated contactor 18.

[0010]There is room for improvement in pre-charge circuits for the direct
current bus of voltage source inverters.

[0011]There is also room for improvement in voltage source inverters
including a pre-charge circuit.

SUMMARY OF THE INVENTION

[0012]These needs and others are met by embodiments of the invention,
which provide a pre-charge circuit for a direct current bus of a voltage
source inverter including a ferro-resonant transformer circuit that
inputs a low voltage, outputs a medium voltage and provides a constant
current source, and a medium voltage diode bridge circuit inputting the
medium voltage and outputting the constant current source to the direct
current bus.

[0013]In accordance with one aspect of the invention, a pre-charge circuit
for a direct current bus of a voltage source inverter comprises: a
ferro-resonant transformer circuit comprising a primary winding
structured to input a low voltage and a secondary winding structured to
output a medium voltage and provide a constant current source; and a
medium voltage diode bridge circuit comprising an input receiving the
medium voltage from the secondary winding of the ferro-resonant
transformer circuit and an output structured to provide the constant
current source to the direct current bus.

[0014]The ferro-resonant transformer circuit may further comprise a
transformer including the primary winding and the secondary winding, and
a capacitor tuning the transformer.

[0015]The ferro-resonant transformer circuit may be structured to disable
the constant current source when the transformer saturates.

[0016]As another aspect of the invention, a voltage source inverter
comprises: an input isolation transformer comprising an input and an
output; a converter comprising an input electrically connected to the
output of the input isolation transformer and an output providing a
direct current bus; an inverter comprising an input electrically
connected to the output of the converter and an output; and a pre-charge
circuit comprising: a ferro-resonant transformer circuit comprising a
primary winding structured to input a low voltage and a secondary winding
structured to output a medium voltage and provide a constant current
source, and a medium voltage diode bridge circuit comprising an input
receiving the medium voltage from the secondary winding of the
ferro-resonant transformer circuit and an output structured to provide
the constant current source to the direct current bus.

[0017]The ferro-resonant transformer circuit may further comprise a
transformer including the primary winding and the secondary winding, and
a capacitor tuning the transformer.

[0018]The ferro-resonant transformer circuit may be structured to disable
the constant current source when the transformer saturates.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]A full understanding of the invention can be gained from the
following description of the preferred embodiments when read in
conjunction with the accompanying drawings in which:

[0020]FIG. 1 is a block diagram in schematic form of a voltage source
inverter including input isolation transformers, contactors, reactors,
rectifier bridge assemblies, a direct current (DC) bus, associated DC bus
capacitors and an inverter.

[0021]FIG. 2 is a block diagram in schematic form of a voltage source
inverter including an input isolation transformer, a converter, a DC bus,
associated DC bus capacitors, an inverter and a pre-charge circuit in
accordance with embodiments of the invention.

[0022]FIG. 3 is a block diagram in schematic form of the pre-charge
circuit of FIG. 2.

[0023]FIG. 4 is a plot of the short circuit characteristic of the
ferro-resonant transformer of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024]As employed herein, the term "number" shall mean one or an integer
greater than one (i.e., a plurality).

[0025]As employed herein, the term "low voltage" shall mean any voltage
that is less than about 600 VRMS.

[0026]As employed herein, the term "medium voltage" shall mean any voltage
greater than a low voltage and in the range from about 600 VRMS to
about 38 kVRMS.

[0027]The invention is described in association with a medium voltage
adjustable frequency drive, although the invention is applicable to a
wide range of voltage source inverters.

[0028]Referring to FIG. 2, a voltage source inverter, such as the example
medium voltage adjustable frequency drive 20 for an example three-phase
motor 21 (shown in phantom line drawing) includes an input isolation
transformer 22, a converter 24, a direct current (DC) bus 26, associated
DC bus capacitors 28,30, an inverter 32 and a pre-charge circuit 34 for
the DC bus 26.

[0030]The pre-charge circuit 34 is preferably not decoupled from the
inverter 32 and the DC bus capacitors 28,30 after those capacitors are
pre-charged. Instead, a contactor 45 is used to apply or remove low
voltage (e.g., without limitation, 120 VAC) power 47 (shown in phantom
line drawing) to the pre-charge circuit 34. A capacitor 46 is used to
tune the ferro-resonant transformer 36, as will be explained.

[0031]The secondary winding 67 of the transformer 36 includes a plurality
of taps 49 and a selector 50 electrically connecting one of the taps 49
to the input 52 of the medium voltage diode bridge 38. These taps 49
provide DC bus voltage selection as needed by the corresponding
application voltage. The plural output tap ferro-resonant transformer 36
may, therefore, be used for different application voltages. The selection
is preferably hardwired, otherwise preconfigured, or made offline with a
suitable selector, such as the example jumper 50.

[0032]A ferro-resonant transformer, such as 36, is very different from a
conventional transformer and provides two very useful output modes.
First, its output waveform to bridge input 52 is very close to a square
wave, and its output amplitude is generally not responsive to the AC
input voltage amplitude of the low voltage power 47. Second, when the
ferro-resonant transformer 36 is short circuited, its output to bridge
input 52 essentially becomes a constant current source.

Example

[0033]Compared to a conventional isolation transformer, a ferro-resonant
transformer, such as 36, has several main differences: (1) a linear
(i.e., non-saturating) AC input (primary) winding, such as 68; (2) a
resonant circuit 35 (e.g., capacitor 46 and secondary winding 67A) with a
saturable magnetic structure (i.e., the core 41 of the transformer 36);
and (3) a magnetic shunt 43 (non-saturating) between the low voltage
input AC power 70 and the saturable magnetic structure, which has the
resonant circuit 35. This saturable magnetic structure is found in close
proximity to the resonant circuit 35. The resonant circuit 35 is tuned
very closely to the frequency (e.g., without limitation, 60 Hz) of the
input low voltage AC power 47. When excited by that frequency, the
resonant circuit 35 builds up enough flux to saturate the (e.g., iron)
core 41 of the ferro-resonant transformer 36 every half cycle. In every
half cycle of operation, the resonant circuit energy is transferred
between the capacitor 46 and the secondary winding inductance.

[0034]The capacitor 46 is electrically connected to the N-turn secondary
winding 67A, whose core (not shown) is constructed with a relatively very
rectangular magnetic material. The resonant frequency of the resonant
circuit 35 is very close to the applied frequency (e.g., without
limitation, 60 Hz) of the input low voltage AC power 47. For example, to
begin a cycle, the capacitor 46 has been charged to potential Em (not
shown) by a current i (not shown) which has left the core (not shown) of
the winding 67A in its negative saturation region (-Os) (not shown). The
voltage Em across the capacitor 46 begins to change the flux from -Os to
+Os (not shown). Only a negligible magnetizing current is required to
produce this change, since the magnetizing curve (not shown) has a
relatively steep slope.

[0035]The capacitor's voltage Em (and, thus, the ferro-resonant
transformer output voltage to bridge input 52), therefore, remains
substantially constant during this period. The flux, changing linearly
with time, reaches +Os in a time τ/2 (not shown), whereupon the
core (not shown) of the winding 67A saturates and the process repeats in
the other direction. Without excitation, oscillations would slowly decay
due to the losses in the circuit. In operation, this energy is supplied
by the AC primary winding 68, which is coupled through the magnetic shunt
43.

[0036]For example, the magnetic shunt 43 is material of the core 41 cut or
otherwise formed to a suitable size to form a relatively lower reluctance
path (or shunt path) for flux. Such core material can be, for example,
layers of a suitable magnetic grade steel. As a non-limiting example, the
AC primary winding (coil) 68 is disposed on the bottom portion (not
shown) of the core 41. The top portion (not shown) of the transformer 36
has on the inside portion (not shown), the secondary winding (coil) 67A
for the tuned capacitor 46, and on the outside portion (not shown) the
high voltage secondary winding (coil) 67 with taps 49. The magnetic shunt
43 is, thus, between the primary winding 68 and the secondary windings
67,67A.

[0037]In an equivalent circuit (not shown) of the ferro-resonant
transformer 36, a resistor RL (not shown) is electrically connected
across the output to the bridge input 52 and represents the circuit load
plus the transformer losses. A linear inductor LL (not shown)
couples the AC input energy to the resonant circuit 35 (capacitor 46 and
parallel inductance LS (not shown)), which is electrically connected
across the output to the bridge input 52. The circuit resonant frequency
is relatively very close to the excitation frequency.

[0038]For a constant input frequency, the capacitor voltage (and, thus,
the output voltage 53 (FIG. 4)) is essentially constant and essentially
independent of the input amplitude of input voltage 70. The input linear
inductance LL (not shown) must be sufficiently high in order to
provide isolation from the input low voltage AC power 47, but low enough
to be able to transfer enough power to maintain resonant oscillation. The
ferro-resonant transformer 36 is also immune to short circuits. Whenever
the output (E) to bridge input 52 is short circuited, the equivalent
circuit essentially becomes the linear inductance LL, which limits
the current to Imax, as can be seen in FIG. 4. This characteristic
gives the very desirable linear charging rate for the DC bus capacitors
28,30.

[0039]Referring again to FIG. 2, the example input isolation transformer
22 includes a three-phase input 54 and a three-phase output 56. The
example converter 24 includes a three-phase input 58 electrically
connected to the three-phase output 56 of the input isolation transformer
22 and a DC output 60 providing the DC bus 26. The example inverter 32
includes a DC input 62 electrically connected to the DC output 60 of the
converter 24 and a three-phase output 64 structured to power a
three-phase rotating electrical apparatus, such as the example motor 21
or a generator (not shown).

[0040]Referring again to FIG. 3, the transformer 36 and the capacitor 46
of the pre-charge circuit 34 form a ferro-resonant transformer circuit
66. The capacitor 46 is electrically connected to the secondary winding
67A and is used to tune the transformer 36 and the ferro-resonant
transformer circuit 66. The capacitor 46 and the core 41 of the
transformer 36 are tuned. The ferro-resonant transformer circuit 66 is
structured to disable the constant current source when the transformer 36
saturates. The transformer 36 includes a primary winding 68 structured to
input a low voltage 70, and the secondary winding 67 structured to output
a medium voltage 72 and provide a constant current source. The primary
winding 68 has a suitable number of first turns, the secondary winding 67
has a suitable plurality of second turns, and a ratio of the plurality of
second turns to the number of first turns is at least about 2400.

[0041]The contactor 45 includes a pair of contactor contacts 73
electrically connected in series with the primary winding 68. The medium
voltage diode bridge 38 includes the input 52 receiving the medium
voltage 72 and the output 44 structured to provide the constant current
source to the DC bus 26 of FIG. 2.

[0042]The converter output 60 of FIG. 2 includes a positive output 74, a
first neutral 76 and a negative output 78. The inverter input 62 includes
a positive input 80 electrically connected to the positive output 74, a
second neutral 82 electrically connected to the first neutral 76, and a
negative input 84 electrically connected to the negative output 78. The
first DC bus capacitor 28 is electrically connected between the positive
input 80 and the second neutral 82 and the second DC bus capacitor 30 is
electrically connected between the negative input 84 and the second
neutral 82.

[0043]The medium voltage diode bridge 38 of FIG. 3 includes a positive
output 86 electrically connected to the positive input 80 of FIG. 2 and a
negative output 88 electrically connected to the negative input 84 of
FIG. 2. A first conductor 90 is electrically connected in series with the
first medium voltage fuse 40 and the positive output 86, and a second
conductor 92 is electrically connected in series with the second medium
voltage fuse 42 and the negative output 88.

[0045]While specific embodiments of the invention have been described in
detail, it will be appreciated by those skilled in the art that various
modifications and alternatives to those details could be developed in
light of the overall teachings of the disclosure. Accordingly, the
particular arrangements disclosed are meant to be illustrative only and
not limiting as to the scope of the invention which is to be given the
full breadth of the claims appended and any and all equivalents thereof.

Patent applications by Irving A. Gibbs, Fletcher, NC US

Patent applications in class Having plural converters for single conversion

Patent applications in all subclasses Having plural converters for single conversion